Industrial processes presently used for urea production are based on the direct synthesis of urea from ammonia and carbon dioxide, according to the following overall reaction:2NH3+CO2→H2N—CO—NH2 
The reaction comprises two consecutive reaction steps, wherein in the first step ammonium carbamate is formed, which is dehydrated in the second step to form urea.
The synthesis reaction leads to the formation of an aqueous solution of urea which needs to be concentrated in order to obtain a urea melt. This melt is further subjected to one or more finishing steps, such as prilling, granulating, pelletizing or compacting. In case of prilling, the urea melt is supplied to a prilling tower wherein it is sprayed from the top of the prilling column in a rising stream of air of ambient temperature in which the droplets solidify to form urea prills.
The prilling process conceals some critical problems including pollution of discharged air with ammonia. Ammonia is typically formed at a high temperature in a urea melt. During the prilling step, the air takes up the formed ammonia which is subsequently released into the atmosphere.
It is therefore desired to reduce ammonia emission in a urea plant and, in particular, to reduce ammonia emission in a prilling tower. It is further desired to be able to reduce ammonia emission of an existing urea plant with minimal alterations to the production process and equipment.